The present invention relates to an ultra-wide field eyepiece for microscopes.
An eyepiece is a sort of loupe, and so has often a simple structure that generally comprises a combination of convex lenses. For this reason, the Petzval's sum assumes a positive value so large that some curvature of field often remains with an increased distortion. In an optical microscope system, the curvature of field is set off by inclining images formed by the objective and eyepiece in opposite directions, thereby achieving a flat image. However, the inclinations of the images are not well corrected. In order to ward off this, reliance is generally placed on some procedure in which the mean image surface of the meridional and sagittal image surfaces is made upright-enough to eliminate the curvature of field.
So far, this procedure, for instance, has been embodied by a lens system set forth in Japanese Provisional Patent Publication No. 60-57315. To be more specific, this system is an external focus type of eyepiece that comprises four lens units, say, in order from the incident side, a positive lens, a negative lens, a positive lens and a positive lens. This eyepiece has a field number as wide as 20 with well-corrected curvature of field, coma, distortion and other aberrations.
Another conventional example of the lens system whose astigmatism is again reduced by erecting the mean image surface is disclosed in Japanese Provisional Patent Publication No. 2-222914. This lens system is an external-focus type of eyepiece that comprises, in order from the incident side, a first lens unit of positive or negative refracting power and a second lens unit of positive refracting power. As in the case of the first example, this lens has a field number as large as 20 with well-corrected aberrations.
Conventional examples similar in lens arrangement to the present invention are set forth in Japanese Provisional Patent Publication No. 3-87805 and Japanese Provisional Patent Publication No. 49-69148. First, the lens arrangement described in Japanese Provisional Patent Publication No. 3-87805 is directed to an eyepiece lens for telescopes that comprises, in order from the incident side, a diverging lens element, a converging lens element and a multi-lens type of converging element located through an intermediate image plane. Japanese Provisional Patent Publication No. 49-69148, on the other hand, relates to an internal focus type of eyepiece for microscopes that comprises five lens units, say, in order from the incident side, a positive lens and a negative lens, a positive lens, a positive lens and a positive lens, the last four lenses located in the rear of an intermediate image plane
While the eyepiece of Japanese Provisional Patent Publication No. 60-57315 is excellent in the flatness of the image surface, some astigmatism remains in the vicinity of the maximum image height having a field number of 20. Moreover, it is of course that the flatness of the image surface gets worse as the field number increases.
The lens system of Japanese Provisional Patent Publication No. 2-222914 is improved in terms of astigmatism, but leaves coma intact. Moreover, the flatness of the image surface gets worse as the field number increases.
The lens system of Japanese Provisional Patent Publication No. 3-87805 cannot stand comparison with an eyepiece for microscopes, because it is an eyepiece for telescopes and so is different from the eyepiece for microscopes in terms of to what degree aberrations are to be corrected.
In addition, the eyepiece for telescopes of Japanese Provisional Patent Publication No. 49-69148 is very unfavorable in terms of coma in the vicinity of the field of view.
As described above, there has not been an eyepiece that is improved in terms of such off-axial aberrations as the curvature of field, astigmatism and coma in an ultra-wide field level that refers to a field of view having a field number of the order of 26.5 until the present invention. Even though a large inclination of the image formed by an eyepiece is set off by the inclination of the image formed by an objective, such a large inclination of the image cannot be well corrected and so is likely to remain. Further, when the meridional image surface is erected so as to correct the inclination of the image, the meridional surface separates from the sagittal surface, leaving some considerable astigmatism intact and giving rise to an image of inferior quality. Still further, when an eyepiece is used in combination with an objective having a large numerical aperture on the exit side, it is impossible to obtain an image of good quality if coma cannot be corrected completely at a place having a large numerical aperture.
When coma remains in the eyepiece, the flatness of the image surface gets worse due to the fact that the exit pupil position varies for each objective or that even with a single objective, the exit pupil position varies by the insertion of an intermediate tube in the rear of the objective.